What is a Breast Pump Flange?

A breast pump flange, also known as a breast shield, is the plastic piece that fits directly over your nipple to form a seal around the areola. This creates a vacuum seal that gently draws your nipple into the flange tunnel for milk extraction. Many pumps come with 24mm flanges, but you may need to select smaller or larger flanges based on your measurement and experience when pumping. 

Having the wrong flange size can affect milk supply, cause nipple pain and damage, and can even lead to clogged milk ducts (ductal narrowing) and mastitis.  Having the correct flange size is also key for allowing adequate pump suction and milk removal.

For first-time pumps, seeing a lactation specialist can be helpful in determining the correct flange size for your nipple size as well as areola density and fullness, and can make a big difference in your pumping experience. 

How to Find the Right Flange Size

  1. Do not pump before measuring, but do stimulate your nipple gently.

  2. Use a measuring tape or silicone flange measuring tool to determine the diameter of your nipple.  

  3. Measure your nipple in millimeters at the widest part (typically the base).
  4. Most moms find that adding 1-3mm to their nipple size provides the most comfortable and effective flange size. 

Available Sizes 

Flanges and sizing inserts (silicone insert that reduces the size of 24mm flanges) are available from 10mm-40mm.  If your flange size isn’t available through your pump’s manufacturer, a Lactation Consultant can guide you in purchasing a flange or sizing insert that is compatible with your pump. 

The COMFY Test

Use the acronym "COMFY" to help determine your flange size. This test was specifically designed to help moms remember the 5 major guidelines for breast shield sizing. 

C – Center Nipples Behind the Flange Opening

Your nipple should generally move in and out of the tunnel without redness or pain. Otherwise, your flange could be too small.

O – Only a Little Areola Tissue Should Be Pulled Into the Tunnel

If your nipple is able to move freely in the tunnel, you may notice a bit of areola pulling into the tunnel with each pumping cycle. If there is no areola movement, your flange might be too small. If there is too much movement, it might be too big.

M – Motion of Your Breast is Gentle and Rhythmic

Your breast movement should be gentle with each pumping cycle, just like your nipple. This indicates your breasts are receiving proper stimulation for expressing breastmilk. 

F – Feels Comfortable Breast Pumping

Your breast pump shouldn't be uncomfortable or cause any pain and tenderness. Discomfort could be caused by the wrong sized pump flange.

Y – Yields Well-Drained Breasts

Your breasts should feel soft and lighter after each pumping session.


During breastfeeding and pumping, the shape and size of your breast and nipple may change. If you start to feel any discomfort or pain with pumping, check the fit or your breast shield/flange. It is possible with the change in the shape or size of your breast/nipple that the breast shield is no longer fitting properly. You should see an International Board Certified Lactation Consultant for an assessment of the fit of the breast shield/flange. Also, check with the manufacturer of your breast pump for information on the available sizes of breast shield and flange.

- Georganna Cogburn -

Will My Flange Size Change?

Possibly! Your breasts may change between the third trimester and after giving birth. Also, your breasts may change through your breast pumping journey, so it's important to check your flange size if your pumping sessions become painful or less effective. 

Because the correct breast shield size is important to express milk and ensure the effectiveness of your pump, you may qualify for new breast pump supplies through insurance. When you receive resupply items you can update your flange size to maintain the correct fit.